During stock preparation in paper manufacture certain paper qualities require that the stock/white water mixture be degassed in an aeration or degassing tank before it is supplied to a headbox via a headbox pump. The degassing takes place by maintaining a negative or sub-atmospheric pressure in the aeration tank by the connection of a vacuum pump thereto, the negative pressure corresponding to the boiling point of the stock at the stock temperature in question and enabling air and other gases to be removed from the stock/white water mixture. The mixture level in the tank is normally controlled by the provision of a spillway in the tank or by the provision of a level regulator which influences a valve in a return conduit. The negative pressure maintained in the aeration tank is normally accomplished by locating the tank at a high level relative to a mixing pump for mixing the stock or paper pulp with white water from a wire pit. This relatively high level of the aeration tank requires long supply and return conduits to be connected to the aeration tank which increases the cost of erecting the paper making plant.
The difference in pressure between that generated across the mixing pump and the negative pressure in the aeration tank must be sufficiently great to overcome the static lifting height between, for example, the levels of liquid in the wire pit and the aeration tank, and the pressure drop in filters, conduits and vortex cleaners between the mixing pump and the aeration tank. The pressure drops across different types of vortex cleaners normally varies between 10 and 30 meters of water gauge (mwg). There are mainly two types of vortex cleaners, namely, one type which works towards a negative pressure and one type which works towards an overpressure. Because of the negative pressure in the aeration tank, all liquid levels to which the aeration tank delivers flows must be located about 10 meters below the liquid level of the aeration tank, which is another reason for the high location of the tank. If the pressure drop across the vortex cleaners is low, which is aimed at, or such vortex cleaners are required which are designed to function towards an overpressure, this high location of the aeration tank is required to prevent stock/white water mixture from the mixing pump and rejects, i.e. dirty material, from the vortex cleaners, from flowing into the aeration tank, thus jeopardizing or making impossible the intended functions.
The present invention aims to provide a method of, or system for, degassing mixtures of first and second materials in an aeration tank in which the above-mentioned drawbacks are substantially eliminated.